1. Field of the Invention
The present invention relates a light emitting element having a layer containing a light emitting substance formed between a pair of electrodes, a light emitting device including the light emitting element, and an electronic appliance.
2. Description of the Related Art
A light emitting element, which has recently been attracting attention as a pixel of a display device or a light source of a lighting device, has a light emitting layer between a pair of electrodes. When current flows between the pair of electrodes, a light emitting substance contained in the light emitting layer emits light.
In development of such a light emitting element, one of important problems is to prevent short-circuiting caused between a pair of electrodes of the light emitting element. One reason of shirt-circuiting caused between a pair of electrodes is a projection generated on each surface of the electrodes. Such a projection is generated when ITO and the like are crystallized, for example. The short-circuiting between the pair of electrodes can be suppressed by reducing roughness of each surface of the electrodes by covering the projection with a thick layer. However, by providing the thick layer, driving voltage of the light emitting element is sometimes increased. Therefore, a technique by which a thick layer is provided without increasing driving voltage has been developed. For example, patent document 1 discloses a technique by which a mixed film in which divanadium pentoxide and α-NPD (note that, α-NPD is also referred to as NPB) are mixed is provided. It is suggested that the short-circuiting can be prevented by providing such a mixed film in the patent document 1.
It is thought that the technique as disclosed in the patent document 1 is extremely effective to reduce short-circuiting caused between a pair of electrodes. However, with respect to absorption spectrum characteristics of the mixed film, in which divanadium pentoxide and α-NPD are mixed, there are large variations in absorption intensity being dependent on an absorption wavelength especially in a visible light region. Accordingly, the amount of light absorbed in the mixed film is varied depending on wavelengths of light emission, thereby causing difference in light extraction efficiency for each wavelength of light emission.    Patent Document 1 : Japanese Patent Application Laid-Open No. 2005-123095